The present invention relates to a method for producing symmetric triglycerides comprising a medium-chain fatty acid at sn (stereo-specific numbering)-1 and 3 positions and a long chain fatty acid at sn-2 position. In particular, the present invention relates to a method for industrially efficiently producing highly pure symmetric triglycerides comprising a medium-chain fatty acid at sn-1 and 3 positions and a long chain fatty acid at sn-2 position.
Various reports were made on methods for producing symmetric triglycerides comprising a fatty acid A at sn-1 and 3 positions and a long chain fatty acid B at sn-2 position in the field of basic research. Methods for producing symmetric triglycerides comprising a long-chain fatty acid as the constituent fatty acid include, for example, a method wherein a triglyceride bonded with oleic acid at the sn-2 position is reacted with sn-1,3 position-specific lipase to produce a triglyceride bonded with oleic acid at the sn-2 position. This method is for producing a cacao butter substitute (see, for example, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as “J. P. KOKOKU”) Nos. Hei 07-83718 and Sho 57-27159, and Japanese Patent Un-Examined Publication (J P Kokai) No. Hei 11-243982). Methods for producing symmetric triglycerides having a polyvalent unsaturated fatty acid at the sn-2 position include, for example, a method wherein an oil or fat containing polyvalent unsaturated fatty acids in a large amount is reacted with a saturated fatty acid and/or an alcohol ester of a saturated fatty acid in the presence of an sn-1,3-position specific lipase (refer to, for example, J. P. KOKOKU No. Hei 07-89944). However, highly pure symmetric triglycerides comprising a medium-chain fatty acid at sn-1 and 3 positions and a long chain fatty acid at sn-2 position cannot be obtained by those methods.
For producing symmetric triglycerides (hereinafter referred to as MLM-type triglycerides) (M: medium chain fatty acid, L: long chain fatty acid) comprising a medium-chain fatty acid at sn-1 and 3 positions and a long chain fatty acid at sn-2 position, for example, the following methods are described in Yugo Iwasaki and Tsuneo Yamane, “Oleoscience” 2001, Vol. 1, No. 8, 825-833:    (1) A method wherein a medium chain fatty acid triglyceride is transesterified with a long chain fatty acid triglyceride in the presence of an sn-1,3-position specific lipase.    (2) A method wherein a long chain fatty acid triglyceride is subjected to an acidlysis reaction with an excess amount of a medium chain fatty acid or a method wherein a long chain fatty acid triglyceride is transesterified with an excess amount of an ethyl ester of a medium chain fatty acid.
MLM triglycerides can also be produced by a two-step reaction as described in, for example, Japanese Patent Unexamined Published Application (hereinafter referred to as “J. P. KOKAI”) No. 2000-270885.    (3) In the first reaction, glycerol and a highly unsaturated fatty acid or a lower alcohol ester thereof is reacted with lipase which is not position-specific to form a highly unsaturated fatty acid triglyceride under dehydration. In the second reaction, the highly unsaturated fatty acid triglyceride and a fatty acid having 12 or less carbon atoms or a lower alcohol ester thereof are reacted with a lipase specifically reactive on sn-1,3-positions to obtain the intended oil or fat having such a structure that the fatty acids bonded at the sn-1 position and sn-3 position have 12 or less carbon atoms and that at least 90% by mass of the fatty acids bonded at the sn-2 position are highly unsaturated fatty acids.
However, according to the method (1), it is difficult to obtain an oil or fat having a high MLM-type triglyceride content because various triglycerides (LML, MLL, LMM, MMM and LLL-type triglycerides) are also synthesized in addition to the MLM-type triglycerides. The term “LML-type triglycerides” indicates symmetric triglycerides having long-chain fatty acids at sn-1,3 positions and a medium chain fatty acid at sn-2 position. “MLL-type triglycerides” indicates asymmetric triglycerides having a medium chain fatty acid at sn-1 position and long chain fatty acids at sn-2,3 positions. “LMM-type triglycerides” indicates asymmetric triglycerides having a long-chain fatty acid at sn-1 position and medium chain fatty acids at sn-2 and 3 positions. “MMM-type triglycerides” indicates triglycerides having medium chain fatty acids at all of sn-1, 2 and 3 positions. “LLL-type triglycerides” indicates triglycerides having long chain fatty acids at all of sn-1, 2 and 3 positions.
In method (2), although it is possible to increase MLM-type triglyceride content by removing the fatty acid and ethyl esters of fatty acids after the completion of the reaction, it is impossible to effectively reuse those removed after the reaction and a high cost is necessitated in the practical production. Thus, symmetric triglycerides have not yet been generally and practically produced.
In method (2), both MLM-type triglyceride and MLL-type triglyceride are obtained after the completion of the reaction. However, it is practically impossible to fractionate these two kinds of triglycerides from each other because they are high-boiling oils or fats. For obtaining MLM-type triglyceride of a high purity by a method different from the fractionation, it is indispensable to use a large amount of ethyl ester of the medium chain fatty acid for the acidlysis reaction. As a result, the productivity is lowered to cause a serious increase in the cost.
In method (3), it is difficult to continuously conduct the first reaction because the enzyme cannot be fed into the column or the like since it is the dehydration reaction. Another defect of method (3) is that a quite excess amount of the lower alcohol ester must be subjected to the second reaction for obtaining the symmetric triglyceride of a high purity.